An electrochemical accumulator usually has a nominal voltage that depends on its type.
For NiMH batteries, the nominal voltage is on the order of 1.2 volts. Batteries based on lead technology have a slightly higher nominal voltage, which is on the order of 2 volts. For lithium ion iron phosphate or LiFePO4, the nominal voltage rises to a value on the order of 3.3 volts. In cobalt-oxide-based lithium ion type batteries, the nominal voltage is on the order of 4.2 volts.
These nominal voltages are too low for the requirements of most systems to be powered. To obtain the appropriate level of voltage, several stages are connected in series. To obtain high values of power and capacitance, several accumulators are placed in parallel in each stage. The number of stages (number of groups of accumulators) and the number of parallel-connected accumulators in each stage will vary according to the voltage, the current, and the capacitance desired for the battery. The association of several accumulators is called “a battery of accumulators.”
Optimizing the use of the battery requires the most precise possible knowledge of its operating parameters.
Among the operating parameters that may turn out to be important for the management of the battery is the battery's state-of-aging, or stage-of-health. The state-of-health represents losses in the battery's capacitance following execution of a sequence of charging/discharging cycles.
There are several ways to define the state-of-aging or state-of-health (SOH(t)) of a battery at an instant t.
A known definition for the state of health of a battery is the ratio of its capacitance at a particular time, t, to its maximum capacitance ex-works, i.e. its maximum capacitance in the condition in which the battery leaves the factory in which it is made. This ratio is given by:SOH(t)=C(t)/C(t0)where t0 represents a time between when the battery leaves the factory and when it is first used. The deterioration of the capacitance C(t) over time characterizes the aging of a battery.
Measuring the capacitance C(t) on a battery requires a full charging/discharging cycle. This rarely happens in common use. For example, in a motor vehicle application, normally one would not allow the battery to be fully discharged since one might be stranded on the road when this happens. It is furthermore necessary to obtain the most frequent possible measurement, especially if the battery goes through microcycles, as is often the case in hybrid vehicles. Microcycles do not enable the performance of a full charging/discharging cycle.
Another known definition does not rely on capacitance. This definition relies on the ratio of the minimum resistance of the battery's output at some initial time t0 to the resistance at time t:SOH(t)=R(t0)/R(t)
The increase in resistance over time also characterizes the ageing of a battery. Since the determination of the battery's state-of-health has to be fairly precise, the resistance of the battery is most often measured in a laboratory, where a well-defined ambient temperature can be made available. Measuring the resistance can be done with current surges at a frequency generally greater than 1 kHz.
Another method for determining aging requires an electrical model of the battery and battery measurements to to determine the parameters of this electrical model. The quality of this aging measurement relies greatly on the precision of the initially chosen electrical model. In addition, the spectral band of measurement is limited by the complexity of the model. Furthermore, since the identification algorithm is complex, it calls for excessive computations for implementation in embedded applications. For embedded applications, there is an unmet need for determining a battery's state-of-health in order, for example, to adapt the operating parameters of an electric motor to the condition of the battery.
Another operating parameter frequently used in battery management processes is the battery's state-of-energy SoE or state-of-charge SoC. Measuring the state-of-charge is most frequently done on the basis of a coulomb count at the end of the charging cycle. However, this measurement undergoes drift and requires a relatively complex readjustment.
Another frequently used parameter is the power available when charging and discharging. Most frequently, these power values are determined from computation charts as a function of temperature and the battery's state-of-charging. Such a determination can prove to be inappropriate for embedded applications.
Another frequently used operating parameter is open-circuit voltage (OCV). The open-circuit voltage charging or discharging value is a DC potential that varies as a function of the state-of-charge and therefore makes it possible to recalibrate the value of the state-of-charge. The determination of the open-circuit voltage relies on an electrical model of the battery and on the capacity of this model to represent the low-frequency behavior of the battery. The definition of the equivalent electrical model of the battery is based on the determining of the impedance of the battery, and forming a non-linear system. Such a determination is made from laboratory measurements. This method is therefore suited for neither an embedded battery nor for independent analysis of different battery modules.
The management of a battery according to the prior art generally comprises an observation of the electrical variables at the output, i.e. at the terminals of the battery, followed by a diagnosis of the battery's operation operation from these observations. This approach proves to be insufficient because it does not enable the precise determination of which zones of the battery, such as which modules or which groups of cells, are, for example, defective or have their own operating parameters. This results in poor optimization of the battery's operation a rapid decrease in its performance when the battery has a defective or less efficient part. This decrease is often accompanied by a rapid aggravation of its condition and by premature aging.
To improve this general approach, there are certain methods of diagnosis that include observing certain parameters of operation at the level of the modules of the battery. This second approach makes it possible to more specifically observe the particular behavior of certain modules of the battery locally. However, its implementation is complex and requires the use of numerous electrical wires to connect a central diagnostic device to each module. This creates major electrical risks because the wires can heat up. In addition, the wires can be bared by friction, which in turn raises the possibility of short circuits, for example between two stages that are relatively distant from each other and have a high difference in potential. Furthermore, this approach requires intermediate galvanic insulation to protect a central computer, the potential of which is associated with its power supply, which can be very distant from the potentials present in the power battery. Finally, this approach enables only unsatisfactory action on the general operation of the battery.